Various methods have been known for braking of AC motors. One common method is to supply DC current (zero frequency current) to the motor. This produces a stationary magnetic field in the motor air gap to oppose rotation. When the spinning rotor interacts with this field, it produces negative braking torque. Such an approach is sometimes called “DC braking,” or “DC injection braking.” In drives (motor controls) where the control functions are performed by a microprocessor executing a stored program of instructions, DC injection adds no additional components and adds little to the cost of the basic drive. However, at high speeds, such as near or above base speed, the available torque may not be as high as desired to bring the motor speed down quickly.
In motors run by inverters with the capacity for operation in a regenerating mode, it is possible to use reverse power flow to provide a negative torque on the spinning rotor. Regenerative braking, using resistors, requires a high current switch which may comprise semiconductors and a resistor of sufficient size to absorb the generated heat.
Steicher, U.S. Pat. No. 6,577,483, illustrates a dynamic braking method where power is returned to the DC bus using a switching brake controller and a power resistor to dissipate power in the form of heat as power is returned to the DC bus during braking.
More recently, there has been interest in dual frequency braking methods. As disclosed in Hammond et al., U.S. Pat. No. 6,262,555, and U.S. Pat. No. 6,417,644, a converter supplies variable frequency AC power to the motor. A first frequency is supplied for normal motoring operation and is summed with a second frequency, a loss-inducing frequency, which is provided when it is desired to produce braking torque. The level of braking can be controlled to generally consume some or all of the braking in the device or motor. As a result of applying these two frequencies simultaneously, further actions must be taken to limit motor pulsation.
Another variable frequency technique is disclosed in U.S. Pat. No. 6,429,612. In this method, using a V/F controller without a current regulation loop, the frequency is reduced by two-thirds from rated operating frequency at synchronous speed and is then again reduced by two-thirds in a second step. This is said to ramp down the speed of the motor while limiting power feedback to that which can be dissipated in the rotor.
The prior art provides braking with relatively high flux remaining in the motor. This can result in tripping of current limit devices. The present invention is aimed at achieving a better control of the braking operation from higher speeds.
The present invention is also aimed at providing the user with a selection of the various types of braking, including several types discussed above.